feat(headless): complete portable single-session host

src/AcDream.Headless/Platform/HeadlessPathSet.cs

@@ -0,0 +1,99 @@
+using AcDream.Headless.Configuration;
+
+namespace AcDream.Headless.Platform;
+
+internal sealed record HeadlessPathSet(
+    string ConfigDirectory,
+    string DataDirectory,
+    string CacheDirectory)
+{
+    internal static HeadlessPathSet Resolve(
+        HeadlessPathOverrides overrides,
+        IHeadlessPlatformEnvironment? platform = null)
+    {
+        ArgumentNullException.ThrowIfNull(overrides);
+        platform ??= HeadlessPlatformEnvironment.Instance;
+
+        string config;
+        string data;
+        string cache;
+        if (platform.IsWindows)
+        {
+            string roaming = RequireFolder(
+                platform,
+                Environment.SpecialFolder.ApplicationData);
+            string local = RequireFolder(
+                platform,
+                Environment.SpecialFolder.LocalApplicationData);
+            config = Path.Combine(roaming, "acdream");
+            data = Path.Combine(local, "acdream");
+            cache = Path.Combine(local, "acdream", "cache");
+        }
+        else
+        {
+            string home = RequireFolder(
+                platform,
+                Environment.SpecialFolder.UserProfile);
+            config = ResolveXdg(
+                platform,
+                "XDG_CONFIG_HOME",
+                Path.Combine(home, ".config"),
+                "acdream");
+            data = ResolveXdg(
+                platform,
+                "XDG_DATA_HOME",
+                Path.Combine(home, ".local", "share"),
+                "acdream");
+            cache = ResolveXdg(
+                platform,
+                "XDG_CACHE_HOME",
+                Path.Combine(home, ".cache"),
+                "acdream");
+        }
+
+        return new HeadlessPathSet(
+            Normalize(overrides.ConfigDirectory ?? config, platform),
+            Normalize(overrides.DataDirectory ?? data, platform),
+            Normalize(overrides.CacheDirectory ?? cache, platform));
+    }
+
+    private static string ResolveXdg(
+        IHeadlessPlatformEnvironment platform,
+        string variable,
+        string fallback,
+        string leaf)
+    {
+        string? configured = platform.GetEnvironmentVariable(variable);
+        string root = string.IsNullOrWhiteSpace(configured)
+            ? fallback
+            : configured;
+        return Path.Combine(root, leaf);
+    }
+
+    private static string RequireFolder(
+        IHeadlessPlatformEnvironment platform,
+        Environment.SpecialFolder folder)
+    {
+        string value = platform.GetFolderPath(folder);
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            throw new HeadlessConfigurationException(
+                $"The operating system did not provide {folder}.");
+        }
+        return value;
+    }
+
+    private static string Normalize(
+        string path,
+        IHeadlessPlatformEnvironment platform)
+    {
+        if (string.IsNullOrWhiteSpace(path))
+        {
+            throw new HeadlessConfigurationException(
+                "Headless directories cannot be empty.");
+        }
+
+        return Path.TrimEndingDirectorySeparator(
+            Path.GetFullPath(path, platform.CurrentDirectory));
+    }
+}

src/AcDream.Headless/Platform/HeadlessPlatformEnvironment.cs

@@ -0,0 +1,28 @@
+namespace AcDream.Headless.Platform;
+
+internal interface IHeadlessPlatformEnvironment
+{
+    bool IsWindows { get; }
+    string CurrentDirectory { get; }
+    string? GetEnvironmentVariable(string name);
+    string GetFolderPath(Environment.SpecialFolder folder);
+}
+
+internal sealed class HeadlessPlatformEnvironment
+    : IHeadlessPlatformEnvironment
+{
+    internal static HeadlessPlatformEnvironment Instance { get; } = new();
+
+    private HeadlessPlatformEnvironment()
+    {
+    }
+
+    public bool IsWindows => OperatingSystem.IsWindows();
+    public string CurrentDirectory => Environment.CurrentDirectory;
+
+    public string? GetEnvironmentVariable(string name) =>
+        Environment.GetEnvironmentVariable(name);
+
+    public string GetFolderPath(Environment.SpecialFolder folder) =>
+        Environment.GetFolderPath(folder);
+}